1. Field of the Invention
This invention relates to a liquid crystal display apparatus and a testing method for a liquid crystal display apparatus.
2. Description of the Related Art
In recent years, reduction of the thickness of apparatus has and is being proceeded rapidly in the field of display apparatus. As thin display apparatus, typically a liquid crystal display (LCD) apparatus has spread widely. The liquid crystal display apparatus is advantageous in that it is thin in thickness, light in weight and low in power consumption, and therefore is frequently used particularly in mobile apparatus such as personal digital assistants (PDA), notebook type personal computers (PC) and portable television receivers. The liquid crystal display apparatus is utilized not only in such mobile apparatus but also in home use television receivers, projectors and so forth.
While an active matrix type and a passive matrix type are available as driving types for a liquid crystal display apparatus, a liquid crystal display apparatus of the active matrix type has become the mainstream in recent years. A liquid crystal display apparatus of the active matrix type has a panel structure wherein two substrates are disposed in an opposing relationship to each other and liquid crystal is filled between the substrates. One of the two substrates has transparent pixel electrodes and thin film transistors (TFTs) formed thereon and is hereinafter referred to as TFT substrate. The other substrate has a single transparent electrode formed on an overall display area thereof and is hereinafter referred to as opposing substrate.
In the active matrix liquid crystal display apparatus, a TFT serving as a switching element is controlled for switching between on and off states for each of pixels disposed two-dimensionally in a matrix to apply a voltage (hereinafter referred to as “gradation voltage”) in accordance with a gradation to the pixel electrode. As a result, a potential difference is produced between the pixel electrode and the electrode on the opposing substrate thereby to vary the transmission factor of the liquid crystal. This is the principle of the liquid crystal display.
On the TFT substrate, a plurality of data lines for supplying a gradation voltage to the pixel electrodes and a plurality of gate lines for applying a control signal for switching the TFTs to the gate of the TFTs are wired in a matrix. Within one frame period of image display, the pixels arranged in a matrix are successively selected in a unit of a column through the gate lines, and a gradation voltage is applied to the pixel electrodes selected in this manner through the data lines thereby to display an image. A gradation voltage applied to each pixel electrode is retained by a capacitive element connected to the output electrode of the TFT until a next gradation voltage is applied.
A liquid crystal display apparatus is frequently formed as a transmission type liquid crystal display apparatus wherein a backlight disposed on the rear side of a liquid crystal panel is used as a light source to irradiate light on the liquid crystal panel from the back side to display an image. On the other hand, a reflection type liquid crystal display apparatus such as an LCOS (Liquid Crystal On Silicon) liquid crystal display apparatus has begun to be placed into the market recently. Since the LCOS liquid crystal display apparatus allows use of a silicon wafer as a substrate, it is advantageous in that transistors having higher performances can be used when compared with transmission type liquid crystal display apparatus wherein circuits are formed from polycrystalline silicon on a glass substrate.
Incidentally, at a stage of fabrication of such liquid crystal display apparatus as described above, a large number of pixels disposed two-dimensionally in a matrix sometimes include a pixel which is rendered defective by some factor. If the number of such defective pixels is excessively great, then normal image display is disturbed. Accordingly, before shipment of a liquid crystal display apparatus, it should be tested in regard to defective pixels. In the testing of pixels, the liquid crystal panel is actually driven, and a display image then is analyzed using an image processing apparatus or is observed by direct visual inspection to decide whether or not the pixels are defective.